Heat a large 12-inch skillet over medium-high heat. Slice the smoked sausage into half-moon pieces and add to the hot skillet. Cook for 5-7 minutes, stirring occasionally, until the edges are lightly browned. Remove the sausage to a plate and set aside, leaving about 1 tablespoon of the drippings in the pan.

Add butter to the same skillet. Once melted, add the diced onion and cook for 3-4 minutes until softened and fragrant. Stir in the minced garlic and cook for an additional 30 seconds until very aromatic.

Add the diced potatoes to the skillet and stir well to coat with the butter and aromatics. Cook for 2-3 minutes, stirring frequently, to lightly brown the potatoes.

Sprinkle the ranch seasoning mix over the potatoes and stir to distribute evenly. Pour in the chicken broth and bring to a simmer. Reduce heat to medium and cook for 8-10 minutes until the potatoes are almost tender, stirring occasionally.

Add the cooked smoked sausage back into the skillet and stir well. Cook for an additional 2 minutes to heat through.

Pour the heavy cream over the potato mixture. Sprinkle both the cheddar and mozzarella cheese over the top, stirring gently until the cheese is fully melted and incorporated. If needed, add a splash more broth if the mixture seems too thick.

Taste and adjust seasoning with salt and black pepper as needed. Remember that the ranch seasoning and sausage already add saltiness, so go easy on additional salt.

Transfer to a serving dish or serve directly from the skillet. Garnish with fresh green onions and parsley if desired. Serve immediately while hot and creamy.